Electrically operated heating element for a hot-runner tool

ABSTRACT

An electrically operated heating element for a hot-runner tool has a heat conductor and is positioned in a hot runner and in runners that derive from the heat conductor and lead to a series of outflow channels 8 from the hot-runner tool. The heat conductor is a flat body with tongues on the side toward the outflow apertures, each tongue extending to the vicinity of one outflow aperture.

This is a division of application Ser. No. 653,990, filed Sept. 24,1984, now U.S. Pat. No. 4,631,389.

BACKGROUND OF THE INVENTION

The present invention relates to an electrically operated heatingelement for a hot-runner tool that has a heat conductor and ispositioned in a hot runner and in runners that derive from the heatconductor and lead to a series of outflow channels from the hot-runnertool.

A hot-runner tool with an electrically operated heating element is knownfrom AT-PS No. 286 607. Its heat conductor is a rod or wire and ispositioned in the center of the tool runner tool where it forms a loopin the vicinity of the runners that derive from the hot runner. Theflanks of the loop extend along the inside of the runners. One drawbackof the known hot-runner tool is that the rod-shaped or wire-shapedconductor can deform as a result of the impact of molten plasticentering the hot runner and come into contact with the surfaces of thehot-runner tool. This contact causes an immediate short circuit anddowntime.

Furthermore, since the heat conductor has the same cross-section at allpoints, the same amount of heat will be generated at every point.

Since the heat conductor forms a loop in the vicinity of one runner andthe loop can only be employed in runners with relatively largediameters, the known hot-runner tool cannot be employed for an injectionmold in which the feed apertures in the mold nests are slightlyseparated and in which small parts are cast.

SUMMARY OF THE INVENTION

One object of the present invention is to provide an electricallyoperated heating element of the aforesaid type whereby the hot-runnertool that is equipped with it can be associated with an injection moldwith a series of mold nests wherein the feed apertures of adjacent formnests are only slightly separated.

This object is attained in accordance with the invention in that theheat conductor is a flat body with tongues on the side toward theoutflow apertures, each tongue extending to the vicinity of one outflowaperture. The free end of each tongue in one practical embodiment is apoint.

The tongues can be in one piece with the flat body or separatecomponents fastened to it. The tongues can be made out of a materialthat is different from the material that the flat body is make out ofand the tongues can have a higher specific electric resistance than theflat body.

There can also be connector plates at each end of the flat body forapplication of an electric voltage and having a lower specific electricresistance than the flat body.

Designing the heat conductor as a flat body and a series of tonguesmakes the heat conductor mechanically strong and dimensionally stableenough to unobjectionably withstand the impact of molten materialentering the hot runner.

Supporting pins made out of an insulating material can rest against thesides of the flat body to position the heat conductor even moresecurely. The supporting pins can be supporting screws that screw intothreaded bores in the hot-runner tool and activated from outside.

Further, the tongues can be between 5 and 15 mm apart and there can berecesses in the flat body above each tongue.

Another object of the invention is to provide an electrically operatedheating element of the aforesaid type wherein the orientation of theheat conductor in relation to the outflow apertures in the hot-runnertool will remain constant subject to the heat expansion that occurs inthe tool during operation and that differs from that of the built-inheat conductor, which will accordingly withstand the impact of moltenmaterial entering the tool.

This object is attained in accordance with the invention in that theheat conductor is composed of U-shaped webs and is positioned betweentwo plates made out of an insulating material and resting against theconductor, which rests on or is fastened to mounts, the ends of whichare positively inserted in mounting recesses in the hot-runner tool.

The U-shaped webs make the heat conductor resilient in shape and it canalso be materially resilient by selecting resilient materials.

The ends of the mounts that the heat conductor rests on or is fastenedto and that are positively inserted in mounting recesses in thehot-runner tool force the conductor to reproduce the motion resultingfrom the heat expansion of the tool. The resiliency of the heatconductor with respect to shape and material allows this without damageto the conductor. The synchronized movement of the heat conductor andthe hot-runner tool subject to the heat stresses that occur duringoperation ensures a constantly precise orientation of the conductor inrelation to the outflow apertures in the hot-runner tool, providing theprerequisites for unobjectionable injection molding.

The plates of insulating material at the center of the heat conductorand resting on the one hand against the conductor and on the otheragainst the inside of the runners in the hot-runner tool reliably securethe conductor along its total length even while molten material isflowing through or around it on the way from the hot runner to theoutflow apertures from the tool.

The plates of insulating material, ceramic for example, form, inconjunction with webs on the heat conductor, flow channels for themolten material.

Other characteristics of the invention will be evident from thefollowing.

Some preferred embodiments of the invention will now be described withreference to the attached drawings, wherein

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial section through a hot-runner tool 1,

FIG. 2 is a section along the line II--II in FIG. 1,

FIG. 3 is a view along the direction indicated by the arrow III in FIG.1,

FIG. 4 is a larger-scale section along the line IV--IV in FIG. 3,

FIG. 5 is a vertical section through another embodiment of a hot-runnertool with an electrically powered heating element,

FIG. 6 is a section along the line VI--VI in FIG. 5,

FIG. 7 is a front elevation of the electrically powered heating elementin FIGS. 5 and 6,

FIG. 8 is a section along the line VIII--VIII in FIG. 7,

FIG. 9 is a partly sectional front elevation of a hot-runner tool withan electrically powered heating element of a different design built in,

FIG. 10 is a section along the line X--X in FIG. 9,

FIG. 11 is a section along the line XI--XI in FIG. 10, and

FIG. 12 is a perspective view of part of the heating element in a largerscale.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As shown in FIGS. 1-4, a hot-runner tool 1 consists of a top 2 and of abottom 3. Top 2 is fastened to bottom 3 with screws 4 and can be removedfrom it. Top 2 has a bushing 5 that demarcates a gate for the moltenplastic. The flow channel constituted by bushing 5 empties into a hotrunner 6, from which runners 7 diverge, each leading to an outflowaperture 8.

Hot-runner tool 1 has a heat conductor 9 with a flat body 10 made out ofa material with a high specific electric resistance. Flat body 10 hasprojecting tongues 11 on the side facing outflow apertures 8. Eachtongue 11 extends into the vicinity of one outflow aperture 8. The freeend of each tongue is pointed. Although tongues 11 can be in one piecewith flat body 10, they can also be separate components attached to theflat body, in which case they can be made out of a different materialthan that of the body. Tongues 11 can have a higher specific electricresistance than flat body 10 to increase the amount of heat generated intheir vicinity.

Also to increase the amount of heat generated in the vicinity of tongues11, each tongue can have recesses 12 or 13 that increase the currentdensity in that vicinity. As will be evident from FIG. 1, the shape ofrecesses 12 or 13 can differ.

In order to concentrate the development of heat in heat conductor 9essentially in the vicinity of flat body 10 and tongues 11, connectorplates 14, which have an essentially lower specific electric resistancethan that of the body and by means of which an electric voltage can beapplied at point 15, are welded to the ends of the flat body in theembodiment illustrated.

As will be evident from FIG. 4, flat body 10 is secured in position withsupporting screws 16 made out of an insulating material, ceramic forinstance, screwed into a threaded bore in hot-runner tool 1, andaccessible from outside.

As will be evident from FIG. 1, since the interval between two adjacenttongues 11 is very narrow, hot-runner tool 1 can be associated with aninjection mold in which the mold-nest gates are separated by the samenarrow interval. Adjacent tongues can be separated by an intervalranging from 5 to 15 mm.

The hot-runner tool 17 illustrated in FIGS. 5-8 has a hot runner 18 thatis supplied with molten material through an inflow aperture 19.

A series of runners 20 diverge from hot runner 18 and lead to an outflowaperture 21 from which the molten material arrives in the mold nest ofan injection mold.

Hot-runner tool 17 has a heat conductor 22a that heats the moltenmaterial both in hot runner 18 and in runners 20. Heat conductor 22a canbe made of a chrome-nickel steel and has connector plates 23 to which anelectric voltage can be applied at each end. The section of heatconductor 22a between connector plates 23 is composed of U-shaped webswith their bottom ends, which face the tool outflow openings or the moldnests, in the form of points 22. Adjacent webs 24 combined into points22 have cross-sections that diminish toward the points. This increasesthe heat output in this area and ensures that the molten material canflow into the injection-mold nest through the outflow aperture in thehot-runner tool subject to satisfactory flow conditions.

The connector plates 23 that accommodate the electric voltage and theheat-conductor webs in the illustrated embodiment have a rectangularcross-section.

Heat conductor 22a is mounted in the hot-runner tool in the vicinity ofconnector plates 23 with bolts 25, which can be made out of aninsulating material. If the bolts are made out of metal their ends mustbe mounted in bushings 26 made out of an insulating material, ceramicfor instance. Bushings 26 are mounted in matching recesses in the tool.

Between connector plates 23 the heat conductor is mounted on bolts 27that engage below transverse webs 28 in heat conductor 22a at the sidethat faces away from the outflow aperture 21 in the tool. These boltscan also be made out of an insulating material or out of metal. In thelatter case the ends of bolts 27 must be mounted as illustrated in FIG.6 in bushings 26 made out of an insulating material.

The section of the heat conductor between connector plates 23 ispositioned between plates 29 that are made out of an insulatingmaterial, ceramic for example, and extend as will be evident from FIGS.5 and 6 over the midsection of the heat conductor but also over thetotal length of hot runner 18. As will be evident from FIG. 6, plates 29contact both heat conductor 22a and the inner surface of the hot runner,providing a secure attachment for the heat conductor. Furthermore,several vertical webs in the heat conductor form, in conjunction withplates 29, flow channels 30 for the molten material to flow throughtoward the associated outflow aperture 21 in the tool.

To intensify heat generated in the vicinity of points 22 it is possibleas illustrated in FIG. 5 to attach the points to a hot wire 31, in whichan electric voltage can be generated.

The hot-runner tool illustrated in FIGS. 9-12 consists of a top 32 and abottom 33 that are mutually centered by means of pins 34 and that arefastened together and can be separated. The top has a connector 35 thataccommodates machine nozzles, through which the molten plastic isintroduced from an introduction opening 36 into a hot runner 37 that hasrunners 38 diverging from it. A hot rod 39 with a point 40 at its freeend extends through each runner. Each hot rod has two separated jaws 41and 42 at the end facing away from the points. Jaws 41 and 42 extendtransversely to the longitudinal midplane 43 of the heating element inwhich the points 40 of hot rod 39 are located.

The adjacent jaws of two hot rods 39 are connected by a bridging web 44that is displaced with respect to longitudinal midplane 43, with eachadjacent bridging web 44 positioned on opposite sides of the midplane.

Jaws 41 and 42 and briding webs 44 are in the vicinity of hot runner 37.To provide the molten plastic being conveyed to the hot runner with anadequate flow cross-section on its way to runners 38, the jaws in theembodiment illustrated in FIGS. 9 through 11 have a central recess 45extending from their upper demarcating surface.

The jaws project laterally beyond the associated hot rod 39 and formlower supporting surfaces 46 and upper supporting surfaces 47 forinsulating bodies 48 and 49, which may be made out of ceramic forexample. Insulating bodies 48 and 49 rest against the inner surface ofthe top and bottom of the tool.

The insulating bodies 48 and 49 in the embodiment illustrated in FIG. 11have a round cross-section and extend over the total length of theheating element.

The heating element is tensioned at its faces between pin-likeinsulating bodies 50, which can also be made out of ceramic.

The heating element is heated by electric resistance. The ends ofheating element have connector components 51 and 52 for a source ofelectric voltage.

Hot rods 39 have flow grooves 53 on two opposing sides. Flow grooves 53extend from the space demarcated by jaws 41 and 42 toward points 40. Themolten plastic flows through the grooves to the gate through therunners.

The displacement of bridging webs 44 with respect to longitudinalmidplane 43 and the empty spaces between jaws 41 and 42 allow the jawsto pivot out into the spaces subject to the increased heat stress thatoccurs during operation so that the heat expansion of the parts of theheat element can be accommodated within the nominal length of theheating element without tensioning of the element generated by theinsulating body being overly stressed.

It will be appreciated that the instant specification and claims are setforth by way of illustration and not limitation, and that variousmodifications and changes may be made without departing from the spiritand scope of the present invention.

What is claimed is:
 1. In an electrically operated heating element for ahot-runner tool that has a heat conductor and is positioned in a hotrunner and in runners that derive from the heat conductor and lead to aseries of outflow channels from the hot-runner tool, the improvementwherein the heat conductor comprises a flat body with tongues on theside toward the outflow channels, each tongue extending to the vicinityof one outflow channel and further comprising recesses in the flat bodyabove each tongue.
 2. The electrically operated heating element as inclaim 1, wherein the free end of each tongue is a point.
 3. Theelectrically operated heating element as in claim 1 or 2, wherein thetongues are integral with the flat body.
 4. The electrically operatedheating element as in claim 1 or 2, wherein the tongues compriseseparate components fastened to the flat body.
 5. The electricallyoperated heating element as in claim 4, wherein the tongues are composedof a material that is different from the material of the flat body. 6.The electrically operated heating element as in claim 5, wherein thetongues have a higher specific electric resistance than the flat body.7. The electrically operated heating element as in claim 1, furthercomprising connector plates at each end of the flat body for applicationof an electric voltage and having a lower specific electric resistancethan the flat body.
 8. The electrically operated heating element as inclaim 1, further comprising supporting pins composed of an insulatingmaterial and resting against the sides of the flat body.
 9. Theelectrically operated heating element as in claim 8, wherein thesupporting pins comprise supporting screws screwed into threaded boresin the hot-runner tool.
 10. The electrically operated heating element asin claim 1, wherein the tongues are between 5 and 15 mm apart.